The operation of spin torque devices is based on the phenomenon of spin torque transfer. If a current is passed through a magnetization layer, called the fixed layer, it will come out spin polarized. This spin (which is angular momentum) will be added to the magnetization in a next magnetic layer, called the fixed layer, and will cause its precession. In the end, if the current exceeds a certain critical value (given by the magnetic material and its environment) the magnetization will be switched by a pulse of current, typically in about 1 nanosecond.